1. Field of the Invention
The present invention relates generally to the field of lipid biochemistry. More particularly, it concerns methods for identification of compounds that stimulate sebum formation, identification of compounds that suppress sebum formation and uses thereof.
2. Description of Related Art
It has long been known that sebaceous gland function in man, and many other animals, is androgen dependent. Many studies have suggested a relationship between the rate of sebum excretion and the severity of androgen dependent disorders. A clear manifestation of this effect of androgens is seen at puberty, where there is an increase in androgen levels in man and a concurrent increase in androgen related acne formation. However it seems that androgen alone is not enough to stimulate sebum formation and some other factor is necessary for androgen to stimulate sebum formation.
Sebum is the holocrine excretion of the sebaceous cell (sebocyte). The maturation of the sebocyte is characterized by lipid production. Histologic evidence indicates that mitotic activity occurs mostly in undifferentiated cells, and that epithelial buds can grow from these cells at any point and differentiate into new acinar units (Brown and Williams, 1972).
From this it has been surmised that once a sebocyte begins to differentiate, it is committed to autolysis. However, the maturational stage at which sebocytes are committed to die has not been experimentally determined in a biologic system. Since maturing sebocytes differentially regulate protein, lipid, and steroid metabolism (Wheatley et al., 1979; Brind et al., 1984; Alves et al., 1986), they may well retain the capacity to proliferate. In support of this concept, the proliferative ability to early differentiating cells on the basis of light microscopy (Plewig et al., 1971).
There have been many attempts to study sebocytes in culture (Wheatley and Brind, 1981). However, studies have been hampered by the limited viability of these cells in culture. Typically the effects of hormones and drugs could only be evaluated in incubations of under 3 hours. Nevertheless, lipogenesis in sebocytes has been found to be stimulated by epinephrine, cyclic adenosine monophosphate (cAMP), and prostaglandin E.sub.2, and it was inhibited by levodopa as well as antilipemic drugs such as nicotinic acid and clofibrate. However, no effect could be elicited from androgens or retinoic acid, apparently because the time-span of hormone exposure was too short to elicit a cellular response.
In the absence of a culture system, the most widely used bioassay system to detect the effects of drugs on sebaceous gland development utilizes the hamster flank organ. This assay is performed in the intact animal and has a major drawback in that androgen application suffices to produce an adequate response in the presence of factors already present within the intact animal. Hence this animal bioassay is completely ineffective for use in determining those factors that augment the effects of androgen in sebum differentiation. Therefore, a clear need exists for a viable cell culture system for the study of sebaceous gland development life cycle and to determine those factors that regulate their maturation and necrosis requires a technique for culturing these cells.
Acne vulgaris is the most common skin disorder in humans. It does not occur in the absence of the increased amounts of androgen that are produced at puberty. Acne results from a combination of increased sebum production, outlet (infundibulum) plugging and secondary infection. It appears that more than androgen is involved in regulating the growth and development of the preputial gland, as in human sebaceous glands (Wheatley, 1986; Thody and Shuster, 1989). This is borne out by several observations: (a) acne wanes after puberty in spite of stable androgen levels; (b) variable expression of androgen action on sebaceous components of pilosebaceous units (PSU) of different body areas (e.g., "beard area" versus "acne area"); (c) variable PSU expression of androgen excess as acne, hirsutism or baldness (Rosenfield, 1986); and (d) failure of androgen to induce substantial sebocyte differentiation in culture. There have been numerous disparate studies as to the identity of the factor that acts with androgen to stimulate sebum production, sebocyte differentiation and ultimately cell death. For example, retinoids cause atrophy and decreased lipid production (Boris et al., 1988; Gomez and Martinez, 1982), antagonizing the effects of testosterone, while catechols stimulate lipogenesis (Wheatley and Brind, 1981).
The reasons set forth above clearly demonstrate the need to elucidate those factors and mechanisms responsible for the stimulation of sebum formation and to find compounds to counteract such stimulation and thereby alleviate the symptoms of sebum formation or indeed to prevent such symptoms form appearing. In order to accomplish these tasks, it is imperative to establish a stable model for the differentiating sebaceous gland.